1. Field of the Invention
The present invention relates to a fluorescent-light image obtaining apparatus for obtaining a fluorescent-light image of the fluorescent-light emitted from a target tissue upon irradiation thereof by an excitation light.
2. Description of the Related Art
There have been proposed technologies for irradiating a target tissue with an excitation light of a wavelength within the wavelength range of the intrinsic tissue fluorophores of the target tissue, and receiving the fluorescent-light emitted from the intrinsic tissue fluorophores of the target tissue thereupon, wherein, utilizing the difference between the fluorescent-light emitted from a normal tissue and the fluorescent-light emitted by a diseased tissue upon irradiation thereof by an excitation light of a predetermined wavelength, a fluorescent-light image of the location of the diseased tissue and its range of lesion is displayed.
Normally, when irradiated by excitation light, as shown by the solid line in FIG. 1, because a strong fluorescent-light is emitted by a normal tissue and a fluorescent-light weaker than that emitted from the normal tissue is emitted from a diseased tissue, as shown by the broken line in the same figure, by measuring the intensity of the fluorescent-light, it can be determined whether the target tissue is in a normal state or a diseased state. However, the fluorescent-light emitted from a target tissue is extremely weak, and because the detection thereof is difficult, as large intensity of fluorescent-light as possible is desirable. However, because there is a fear that injury to the patient result from too strong an excitation light, it must be controlled to be of a uniform intensity below a certain level. Levels of intensity of excitation light that do not cause injury to a patient are defined as MPE values according to the JIS standard, etc. Further, because the excitation light is spread at an angle of 100° at the excitation light emitting end of the endoscope insertion portion, as shown in FIG. 2, the relationship of the distance between the distal end of excitation light emitting means and the target area to the intensity of the excitation light received at the target area is such that the intensity of the excitation light becomes greater as the distance becomes shorter. Accordingly, the distance between the distal end of excitation light emitting means and the target area facilitating operation below the MPE value of 2000 W/m2 shown in FIG. 2 is 3 mm or more.
However, in using a fluorescence endoscope apparatus, etc., because the target tissue is a tube-shaped organ, the excitation light emitting end of the endoscope insertion portion cannot be fixed in place and the distance between the target area an the excitation light emitting end of the endoscope insertion portion is not uniform. Therefore, when the intensity of the excitation light has been set close to the MPE value in order to obtain adequate fluorescent-light, if the distance between the excitation light emitting end of the endoscope insertion portion and the target area becomes less than 3 mm, it is possible for the target tissue of the target area be injured. On the other hand, under all measurement-taking conditions, including cases in which the distance between the excitation light emitting end of the endoscope insertion portion and the target area becomes less than 3 mm, for cases in which the intensity of the excitation light has been set so as to ensure for the safety of the target tissue of the target area, at the far end of the normal operational distance range (50-100 mm), the intensity of the excitation light becomes weak and an adequate intensity of fluorescent-light is not obtained, whereby the accuracy of the detection is reduced. Further, for cases in which the sensitivity of the detection system has been increased in order to detect such weak levels of fluorescent-light, the increase in the cost of the system is extremely high.